The present invention relates to a power transmission control system used for a vehicle whose engine is controlled to allow termination of fuel supply and whose driving force is transmitted through a power-transmission mechanism to drive wheels by a transmission control being executed with a frictionally engaging element (for example, friction clutch).
Generally, an automobile includes a construction for transmitting the driving force of an engine to drive wheels through a power-transmission mechanism for driving a vehicle body. Additionally, for the purpose of improving fuel economy, a vehicle, nowadays, is equipped with a fuel-supply termination control (fuel cut-off control), in which the supply of fuel to the engine is stopped when the vehicle is in a deceleration with the throttle being closed (i.e., the accelerator pedal being released). However, it is known that if the fuel supply is terminated at the same time as the throttle is closed, then an abrupt engine brake may be generated to create a negative torque on the drive wheel side, resulting in a deceleration shock. To solve this problem, before the supply of fuel is terminated (for example, when the throttle is determined to have closed), the power-transmission clutch (frictionally engaging element) that is in operation establishing a power transmission path in the power-transmission mechanism currently is disengaged, or the lock-up clutch (frictionally engaging element) of the torque convertor is released to reduce the deceleration shock. In this case, the frictionally engaging element such as the power-transmission clutch or the lock-up clutch, having been disengaged, is engaged gradually again after the elapse of a predetermined time while the fuel cut-off is continued, achieving an improvement in both fuel economy and drive performance.
However, a vehicle in this fuel-supply termination control has a weakness. While the vehicle is traveling at a low speed, if the accelerator pedal is operated to return slightly, then this action may be taken as a closing of the throttle in this control, resulting in a termination of the fuel supply. This misjudgment in the control can happen because the opening of the throttle is small while the vehicle is traveling slowly. If such a misjudgment happen, this will result in a deceleration that can be against the driver""s will. To prevent such a situation from occurring, a predetermined delay is provided between the time points when the throttle is determined to have closed and when the fuel supply is terminated. This delay is variably adjusted in correspondence to the condition of the vehicle in traveling.
In this case, where the delay before the termination of the fuel supply is adjusted variably, it is difficult to determine an appropriate time at which the frictionally engaging element (power-transmission clutch or lock-up clutch), having been disengaged, should be engaged again, because a conventional practice has been that the time for the clutch to re-engage (or the time period during which the clutch is disengaged) is set at a predetermined time on a timer. If the time period for disengaging the clutch is too short, then the timing of the termination of the fuel supply and that of the re-engagement of the clutch are too close to each other, so the result will be a deceleration shock. Conversely, if the time period during which the clutch is disengaged is relatively long, then the rotational speed of the engine will decrease rapidly to a level at which the termination of the fuel supply is not allowed, or even if the fuel supply is terminated, it will be started again in a short time. In this situation, it is difficult to improve the fuel efficiency of the vehicle.
This problem is more serious in a hybrid-type power transmission, which incorporates an electrical motor generator provided in parallel with the engine to recover energy by exploiting the rotational drive of the drive wheels during the deceleration of the vehicle (by generating electricity with the electrical motor generator and charging the battery). This is because, during the deceleration of the vehicle, the torque driving the electrical motor generator acts to further decelerate the vehicle.
In this type of power transmission, which recovers energy by the electrical motor generator driven from the deceleration of the vehicle, generally, if the brake is activated to restrict the rotation of the wheels during the deceleration, the torque driving the electrical motor generator (referred to as xe2x80x9crecovery torquexe2x80x9d) is increased to increase the rate of energy recovery by the electrical motor generator. This recovery torque increases the rate of energy being recovered and, at the same time, assists the braking of the drive wheels to unburden the braking system. In this control, for example, in a case where the brake pedal is stepped down simultaneously while the accelerator pedal is released to close the throttle, if the clutch is released at the same time as the throttle is closed, then a large deceleration torque (negative torque) can act on the clutch, increasing the clutch slip ratio. As a result, the rotational speed of the engine decreases rapidly, this condition preventing the termination of the fuel supply. In such a case, even if the fuel supply is terminated, the fuel supply is restarted in a short time. After all, there is no gain in fuel economy. Furthermore, if the clutch slip ratio increases rapidly, then the clutch is controlled to engage quickly to decrease the clutch slip ratio. This may result in a shock and impair the driving performance of the vehicle.
Another problem is that if the rotational speed of the engine decreases rapidly during a deceleration, then it can happen that the recovery torque be controlled to increase, resulting in a vicious spiral of the rotational speed being further decreased. This can happen because a vehicle with a hybrid-type power transmission is often equipped with a recovery control that sets the rate of energy recovery to a predetermined power (KW) for the purpose of stabilizing the deceleration.
It is an object of the present invention to provide a power-transmission control system for a vehicle, which system improves fuel economy by controlling the termination of the fuel supply while the vehicle is in a deceleration with the throttle being closed.
It is another object of the present invention to provide a power-transmission control system for a vehicle, which system controls the clutch to engage smoothly without a shock while the vehicle is in a deceleration with the throttle being closed.
To achieve this objective, the present invention provides a vehicular power-transmission control system that comprises an engine, a power-transmission mechanism (for example, the continuously variable transmission CVT described in the following embodiment) and a frictionally engaging element (for example, the starting clutch 5, the forward clutch 25 and the reverse brake 27 described in the following embodiment). The engine can be operated under a fuel-supply termination control, and the power-transmission mechanism transmits a rotational driving force from the engine to wheels. The frictionally engaging element controls the transmission of the rotational driving force in the power-transmission mechanism. When the throttle of the engine is closed to decelerate the vehicle in traveling, this control system executes a vehicle-in-deceleration control by terminating the fuel supply to the engine at the time when a predetermined time has elapsed since the starting of the deceleration. In the vehicle-in-deceleration control, when the throttle is detected to have closed, the engaging force of the frictionally engaging element is set to reduce the torque being transmitted through the frictionally engaging element. However, after the termination of the fuel supply to the engine, the engaging force is gradually increased to bring the frictionally engaging element gradually into engagement.
According to this vehicular power-transmission control system, when the throttle of the engine is closed to decelerate the vehicle in traveling, the control system at first detects the closing of the throttle and releases the frictionally engaging element into a pre-engagement condition. As a result, there is no possibility for the drive wheels to receive an abrupt engine brake, so the possibility of deceleration shock is eliminated completely. Thereafter, the control system waits for a predetermined time to elapse and then terminates the fuel supply to the engine for improving fuel economy. After the termination of the fuel supply, the control system increases the engaging force of the frictionally engaging element gradually. This action securely prevents both a rapid decrease in the rotational speed of the engine and a shock that may otherwise occur by an action of a large decelerating force, which is generated in relation to the termination of the fuel supply. According to the present invention, once the fuel supply is terminated, unlike a prior-art control system, there is no chance for the fuel supply to be restarted in a short time by a rapid decrease in the rotational speed of the engine. Therefore, the fuel-supply termination control is executed in a desirable way to improve the fuel economy of the vehicle.
In the vehicle-in-deceleration control, if the frictionally engaging element slips to the decelerating side, it is preferable that the engaging force of the frictionally engaging element be controlled to increase gradually and to restrict the slip ratio within a range smaller than a predetermined value. This action securely prevents a rapid decrease in the rotational speed of the engine. Therefore, unlike a prior-art control system, the control system according to the present invention gives no chance for the rotational speed of the engine to decrease rapidly triggering a restarting of the fuel supply in a short time after the termination of the fuel supply. This condition enables the fuel-supply termination control to be executed in a desirable way to improve the fuel economy of the vehicle.
Furthermore, the control system may comprise an electrical motor generator, which is provided at a position on the side of the engine from the frictionally engaging element. In this case, the electrical motor generator can drive the drive wheels through the power-transmission mechanism, and it can also recover energy by receiving a driving force from the drive wheels through the power-transmission mechanism. In the vehicle-in-deceleration control, the control system preferably sets the electrical motor generator to receive the driving force of the drive wheels through the power-transmission mechanism and to recover energy in correspondence to the engagement condition of the frictionally engaging element. In this case, while the energy recovery is going on, if the rotational speed of the engine become equal to or lower than a predetermined rotational speed, then preferably, the energy recovery rate is controlled to become lower.
If a system is arranged with an electrical motor generator to perform energy recovery as described above, customary, the rate of energy recovery is set to a predetermined power (KW) for the purpose of stabilizing the deceleration. The control system according to the present invention reduces the energy recovery rate if the rotational speed of the engine is lower than a predetermined rotational speed as described above. By controlling in this way, the control system prevents the rotational speed of the engine from decreasing. For example, when the brake is operated to restrict the rotation of the wheels during a deceleration of the vehicle, although the energy recovery rate of the electrical motor generator is controlled to increase, if the rotational speed of the engine decreases rapidly, then the energy recovery rate is controlled to decrease so that the rotational speed will not become equal to or lower than the predetermined rotational speed. In this way, a rapid decrease is avoided in the rotational speed of the engine.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.